Plasma processing methods and apparatus

ABSTRACT

To move an article in and out of plasma during plasma processing, the article is rotated by a first drive around a first axis, and the first drive is itself rotated by a second drive, so that the article enters the plasma at different angles for different positions of the first axis. The plasma cross-section at the level at which the plasma contacts the article is such that those points on the article that move at a greater linear velocity (due to being farther from the first axis) move longer distances through the plasma. As a result, the plasma processing time becomes more uniform for different points on the article surface. The direction of rotation of the first and/or second drive changes during processing to improve processing uniformity. The article is allowed to be processed with the plasma only during one-half of each revolution of the second drive. In the other half of each revolution, the processing is substantially prevented by increasing the second drive velocity as the article is carried through the plasma.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to processing of materials, and moreparticularly to plasma processing.

Plasma processing is widely used to modify surface properties ofmaterials. Thus, plasma is used in fabrication of integrated circuits toperform deposition, etch, cleaning, and rapid thermal anneal.Plasma-based surface processes are also used for hardening of surgicalinstruments and machine tools, and are used in aerospace, automotive,steel, biomedical, and toxic waste management industries. See, forexample, M. A. Lieberman and A. J. Lichtenberg, “Principles of PlasmaDischarges and Materials Processing” (1994), page 1.

A common goal in a plasma-based process design is uniform treatment ofthe target surface (i.e. the surface treated with plasma). It isdesirable to develop systems in which the uniform processing isfacilitated.

In some systems, the target article and the plasma move relative to eachother, and it is desirable to facilitate precise control of thisrelative movement. Further, it is desirable to reduce stresses on thetarget articles thus reducing the possibility of damaging the targetarticles.

SUMMARY

Some embodiments of the present invention provide methods and apparatusfor moving the target articles relative to the plasma so as tofacilitate uniform processing of the target surfaces. In particular,some embodiments facilitate precise control of the movement of thearticles relative to the plasma by reducing accelerations of thearticles. Reducing the accelerations also results in reduction ofstresses to which the articles are subjected.

As the target article moves through the plasma, the article velocity mayhave to be varied to achieve uniform plasma processing. Consider, forexample, the dynamic plasma treatment (DPT) system described in Yu. M.Agrikov et al., “Dynamic Plasma Treatment of HIC (Hybrid IntegratedCircuit) Substrates”, Elektronnaya Tehnika, Ser. 10, 5(71), 1988, pages30-32, incorporated herein by reference. In that system, a targetsubstrate is moved in and out of the plasma in a chamber maintained atatmospheric pressure. The substrate is moved by a horizontal armrotating in a horizontal plane. The plasma flows vertically,intersecting the substrate path. The horizontal cross-section of theplasma is smaller than the substrate surface being treated. Therefore,the plasma source moves along the rotation radius to process the wholesurface.

Since the substrate points that are located farther from the rotationaxis move faster than the points closer to the rotation axis, the pointsfarther from the rotation axis could be exposed to the plasma for lesstime than the points closer to the axis, resulting in non-uniformprocessing. One solution to this problem is to vary the angular velocityof the substrate as the plasma source moves along the rotation radius.Thus, when the plasma source is farther from the rotation axis, theangular velocity can be decreased to increase the time that thesubstrate moves through the plasma.

Another solution is to vary the velocity of the plasma source.

Both solutions need improvement. Thus, varying the angular velocity ofthe substrate leads to accelerations that make precise control of theangular velocity more difficult to achieve. Further, these accelerationscreate stresses that may damage the substrate if the substrate isfragile, for example, if the substrate is a semiconductor wafer.Therefore, for this solution, it is desirable to reduce variations ofthe substrate angular velocity.

Varying the velocity of the plasma source is disadvantageous becauseaccelerations experienced by the plasma relative to immobile ambient gascan change the plasma characteristics and hence make the processing lessuniform. Of note, if the processing occurs at atmospheric pressure (asdoes DPT), even constant-velocity movement of the plasma source can makethe plasma difficult to control unless the plasma motion is very slow.Thus, it is desirable to reduce the velocity and acceleration of theplasma source, preferable down to zero.

Accordingly, in some embodiments of the present invention, targetsurface points that move at different velocities are caused to traveldifferent distances through the plasma so that the faster moving pointstravel a longer distance. As a result, the time spent in the plasma byfaster moving points approaches the time spent by slower moving points.Consequently, the accelerations needed to make the plasma processinguniform are reduced.

In some embodiments, the plasma source is stationary.

In some embodiments, these advantages are achieved as follows. Theplasma flow cross-section through which the target article moves is madeto have different dimensions in different directions. The target articlepasses through the plasma multiple times in different directions so thatthe points moving faster intersect the plasma along a longer dimensionof the cross-section than the slower moving points. As a result, uniformtreatment can be obtained with less variation of the article velocity.

In some embodiments, the plasma source is stationary. Changing thedirection in which the target article intersects the plasma is achievedby rotating the drive that rotates the article so that the articlerotates around a first axis which itself rotates around a second axis.The directions change because the article intersects the plasma atdifferent positions of the first axis.

In some embodiments, the article rotates, and the direction of rotationis changed during processing. When plasma processing takes place at ahigh pressure (for example, atmospheric pressure), plasma becomesunstable when the article enters the plasma. As a result, the articleedge points that enter the plasma are processed at a lower rate than therest of the article. Changing the direction of the rotation helpscompensate for this non-uniformity. In some embodiments, the articlerotates around a first axis, and the first axis rotates around a secondaxis. Either the rotation around the first axis or the rotation aroundthe second axis, or both, can change direction.

In some embodiments, the article rotates around the first axis and thefirst axis rotates around the second axis. During one half of eachrevolution around the second axis, the article is processed with theplasma. During the other half of each revolution around the second axis,plasma processing of the article is substantially prevented. In someembodiments, the processing is substantially prevented by increasing thevelocity of the article to cause the article to move so quickly throughthe plasma that substantially no processing has time to occur. In someembodiments, this further improves processing uniformity by allowingdifferent portions of the article to heat and cool more uniformly.

Other embodiments and variations are discussed below. The invention isdefined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plasma processing system according tothe present invention.

FIGS. 2-4 are top views illustrating wafer trajectories in the system ofFIG. 1.

FIG. 5 is a top view illustrating different positions of plasma relativeto a wafer in the system of FIG. 1.

FIG. 6 shows velocity graphs for the system of FIG. 1 and for a priorart system.

FIG. 7 is a top view illustrating different positions of a waferrelative to the plasma in the system of FIG. 1.

FIGS. 8-12 are top views illustrating different wafer trajectories inthe system of FIG. 1.

FIG. 13 illustrates wafer and carousel dimensions in the system of FIG.1.

FIGS. 14 and 15 show velocity graphs for the system of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

In plasma processing system 110 (FIG. 1) plasma source 114 generatesplasma jet 120 schematically shown by an arrow. Plasma jet 120 flowsvertically upwards through an elongated opening 114-O in source 114. Theopening 114-O is elliptical in some embodiments. Horizontal crosssections of plasma jet 120 are also elliptical. In some embodiments, theopening 114-O and the plasma horizontal cross sections have a shape ofan elongated rectangle, perhaps with rounded corners.

Carrousel 124 includes five holders 130. Each holder 130 holds anarticle 134 (FIG. 2) whose bottom surface is processed by plasma jet120. Articles 134 in FIGS. 1-5 are round semiconductor wafers whose flathorizontal bottom surfaces are processed with plasma 120. In someembodiments, the articles are not round and/or are not semiconductorarticles. In some semiconductor-wafer embodiments, holders 130 arenon-contact wafer holders such as described, for example, in PCTpublication WO 96/21943 “Device For Treating Planar Elements With APlasma Jet” published July 18, 1996 (inventors I. M. Tokmulin et al.)incorporated herein by reference. In some such embodiments, the plasmaprocessing takes place at atmospheric pressure or at some other pressureclose to the atmospheric pressure.

In some embodiments, wafer holders 130 hold the wafers by vacuum or byelectrostatic, mechanical, or some other means.

Some embodiments of system 110 have only one older 130, or some othernumber of holders.

Each wafer holder 130 is attached to an arm 140A of a first angle drive140. Angle drive 140 rotates the wafers around vertical axis 140X.

Angle drive 140 is attached to arm 150A of second angle drive 150. Drive150 rotates around vertical axis 15OX.

Control system 154 controls the drives 140 and 150 by conventional means(for example, using a computer).

Top view FIGS. 2-4 show different positions of a single wafer 134 duringprocessing.

In some embodiments, plasma source 114 does not move during processing,and therefore the plasma jet is stationary, that is, the plasma jetfills a substantially stationary region in space. However, the plasmasource can move vertically and horizontally before processing in orderto be positioned at a desired location relative to targets 134 (and, inparticular, at a desired distance relative to the targets) as dictatedby processing requirements.

Some embodiments of the plasma source are described in PCT applicationWO 98/31038 (published Jul. 16, 1998) entitled “Plasma Generation andPlasma Processing of Materials”, incorporated herein by reference. Seealso PCT publications WO 92/12610 (published Jul. 23, 1992), WO 92/12273(published Jul. 23, 1992), WO 96/21943 (published Jul. 18, 1996), whichare incorporated herein by reference.

In some embodiments, the angular velocity W1 of drive 140 is constantand thus is simple to control. Angular velocity W2 of drive 150 isconsiderably smaller than W1. In some embodiments, the average angularvelocity of drive 150 is ten times smaller than the velocity W1.Therefore, wafers 134 cross the plasma jet multiple times during eachrevolution of drive 150.

In some embodiments, system 110 is used for dynamic plasma treatmentperformed at atmospheric pressure such as described, for example, in Yu.M. Agrikov et al., cited above, incorporated herein by reference. Theangular velocities W1 and W2 are controlled so that the linearvelocities of points being treated with plasma are greater than thespeed at which the heat from the plasma propagates through article 130.Consequently, the processing conditions approach the conditions thatwould exist if the entire target surface were simultaneously exposed tothe plasma. In some embodiments described, W1 is a constant velocity ofabout 5 to 30 revolutions per second, and W2 is much smaller, theaverage value of W2 being at least 10 times smaller than W1 in someembodiments.

Dynamic plasma treatment is also described in the following articlesincorporated herein by reference: P. P. Kulik, “Dynamic Plasma Treatment(DPT) of a Surface of a Solid Body”, Plazmohimiya-87, Part 2 (U.S.S.R.Academy of Science, Institut Neftehimicheskogo Sinteza im. A. V.Topchieva, Moscow, 1987), pages 4-13; Yu. M. Agrikov et al.,“Foundations of a Realization of a Method of Dynamic Plasma Treatment ofa Surface of a Solid Body” (same publication, pages 58-96.)

In some embodiments the plasma processing is performed in vacuum.

FIGS. 1-4 use the following notation:

Numeral 160 denotes a vertical axis that passes through the center ofthe opening 114-O and plasma jet 120. Axis 160 is a symmetry axis of theopening 114-O, the plasma jet 120, and plasma source 114.

R1 is the distance between axis 140X and the nearest edge point 134C ofwafer 134 (all the distances in FIGS. 2-4 are taken between the parallelprojections of respective points onto a horizontal plane unlessmentioned otherwise).

LS is the distance between the axis 140X and the farthest edge point134F of wafer 134.

R2 is the distance between the axes 140X and 150X.

LP is the distance between axis 150X and axis 160 of plasma jet 120.

120F denotes the horizontal cross section of plasma jet 120 at the levelof the lower surface of wafer 134. This cross section is called a“plasma footprint” below. The long axis of this elliptical footprint isshown at 210; the short axis is shown at 214. Axis 214 is perpendicularto axis 210.

Short axis 214 lies on axis 220 which intersects the axes 160, 150X inthe top view of FIGS. 2-4.

P1 is the length of footprint 120F, and P2 is the width of thefootprint.

In FIGS. 2-4, each circle 234C is a trajectory of the wafer point 134Cfor some position of the axis 140X. Each circle 234F is a trajectory ofpoint 134F for some position of axis 140X. The actual trajectories ofpoints 134C and 134F are not circles because the axis 140X rotatesduring processing. However, in embodiments in which the angular velocityW2 of drive 150 is small relative to the angular velocity W1 of drive140, circles 234C, 234F are fair approximations of the actualtrajectories of points 134C, 134F.

In FIG. 2, circle 240 is the smallest circle circumscribed around allthe circles 234F.

Angles Θ (Θ1 in FIG. 3 and Θ2 in FIG. 4) are angles between the axis 220and arm 150A (that is, between axis 220 and a horizontal straight lineintersecting the vertical axes 15OX and 140X). In FIG. 2, Θ=0. Angles Ψ(Ψ1 in FIG. 3, Ψ2 in FIG. 4) are angles between (1) the long axis 210 ofplasma footprint 120F and (2) the trajectory of a selected point onwafer 134 where this trajectory intersects the axis 210. The selectedpoint is chosen on the diameter 134D interconnecting the points 134C and134F. In FIG. 3 (Θ=Θ1), this selected point is 134F. In FIG. 4, theselected point is 134C. Different wafer points are selected fordifferent angles Θ because no single wafer point intersects the plasmafootprint at every value of Θ. Of note, in some embodiments, the waferdiameter is 100-300 mm; the length P1 of the footprint 120F is 50-150mm; the footprint width P2 is 20-50 mm; R1 is about 20-100 mm; R2 is150-300 mm; LS is 120-400 mm; LP is 150-300 mm.

The trajectory determining the angle Ψ is drawn assuming the arm 150A isstationary. The drawn trajectory is a good approximation of the actualtrajectory if the angular velocity W2 of drive 150 is small relative tovelocity W1 of drive 140.

In FIG. 2, wafer 134 does not intersect plasma footprint 120F.

As illustrated in FIGS. 2-5, as the arm 150A approaches plasma jet 120(the angle Θ increases), the intersection of plasma footprint 120F withthe wafer 134 approaches the point 134C and axis 140X. In FIG. 5, theellipse 120F1 shows the position of plasma footprint 120F relative towafer 134 when Θ=Θ1 (FIG. 3). In that position, the plasma footprintcovers the point 134F. The ellipse 120F2 shows the plasma footprintposition at Θ=Θ2 (FIG. 4). In that position, plasma footprint 120Fcovers the point 134C. Ellipse 120F3 shows the position of the plasmafootprint 120F when Θ has an intermediate value between Θ1 and Θ2.

As the plasma footprint moves closer to point 134C and to axis 140X, theplasma processes wafer points having lower linear velocities. Indeed,the linear velocity relative to axis 140X (corresponding to the angularvelocity W1) decreases because the distance from axis 14OX decreases.Since angular velocity W1 is considerably higher than W2, the linearvelocity component corresponding to W1 dominates the point's resultantlinear velocity. In addition, as the angle Θ increases (see FIGS. 2-4),the angle between the vector of the linear velocity relative to axis140X and the linear velocity of axis 140X relative to axis 15OXincreases for points passing through the plasma. The increasing angletends to further reduce the magnitude of the resultant linear velocity.

The decreasing linear velocity tends to increase the plasma processingtime for points closer to the point 134C. However, the decreasing linearvelocity is at least partially offset by the decreasing length of thepoints' trajectories through the plasma footprint. For example, thetrajectory T2 of point 134C through ellipse 120F2 is shorter than thetrajectory T3 of the point passing through the center of ellipse 120F3.The trajectory length decreases because the plasma footprint turnsrelative to the wafer so that as Θ increases from 0 to 180°, the anglebetween short axis 214 of the plasma footprint and the arm 140Aincreases from about 0 towards 90°. (In FIG. 5, the axis 214 is shown at214-1, 214-2, 214-3 for respective plasma footprint positions 120F1,120F2, 120F3. The long axis 210 in the position 120F2 is shown at210-2.) Since the wafer points travel through the footprint essentiallyat 90° to arm 140A, their trajectories become shorter.

In some embodiments, the angular velocities W1 and W2 are chosen so thatas Θ increases to 180°, each point on wafer 134 passes through footprint120F several times during several successive revolutions of first drive140. Thus, successive plasma paths on the wafer surface overlap. WhenΘ=0 or Θ=180°, wafer 134 does not pass through plasma 120. Therefore,points 134C and 134F pass through the plasma about the same number oftimes as every other wafer point.

To reduce the probability that different points may pass a differentnumber of times through the plasma, in some embodiments each article 130is processed in two or more revolutions of drive 150. In eachrevolution, the plasma traces a different path on the article surface,thus increasing the processing uniformity. To obtain different plasmapaths in different revolutions, the following techniques are used:

I. After each revolution, the drive 150 is stopped for a while in theposition Θ=0 (FIG. 2), while the drive 140 continues to rotate. Thendrive 150 is restarted at a time when the position of wafer 130 isdifferent from the wafer position at the start of a previous revolution.In some embodiments, the stopping time is a few milliseconds.

II. Alternatively, drive 150 is not stopped at Θ =0, but the velocity W2is changed near Θ=0 compared with a previous revolution (for example, W2is increased or decreased by 0.1% when Θ is near 0), so that when Θincreases to a value at which the wafer 130 starts intersecting theplasma 120, the wafer 130 has a different position from its position forthe same Θ in a previous revolution. Such small variations of W2 can beperformed with less acceleration of drive 150 than in option I. Further,since drive 150 is not stopped, the processing time is less than inoption I. In some embodiments, for Θ near 0, W2 is increased for 2 or 3revolutions of drive 150, then W2 is decreased for a few revolutions ofdrive 150, then increased again. In some embodiments, for Θ values atwhich the wafers 130 intersect the plasma, W2 is the same in eachrevolution.

III. Velocity W1 of drive 140 is varied slightly (for example, by 0.1%)between different revolutions of drive 150.

The technique III is combined with I or II in some embodiments.

In some variations of techniques I, II, III, W2 and/or W1 are variedwhen Θ is near 180° and/or 0° and/or some other value.

The technique I has the advantage of allowing the wafer 130 to cool atleast part of the way down to its original temperature after eachrevolution of drive 150, thus allowing the thermal conditions to besimilar at each revolution. See PCT Publication WO 96/21943 publishedJul. 18, 1996, incorporated herein by reference. In some embodiments,the wafer is stopped for a few seconds at Θ=0 to allow the wafer tocool. If the processing is sensitive to the thermal conditions, velocityW1 is controlled so that the wafer is allowed to cool during eachrevolution of drive 140.

In FIGS. 2-4, the following relations hold true:

 R 1≧P 1/2  (1)

that is, the distance R1 between the axis 140X and the nearest edgepoint 134C of wafer 134 is greater than or equal to one half of thelength P1 of plasma footprint 120F.

This condition ensures in any revolution of first drive 140, any givenpoint on wafer 134 passes through plasma 120 at most once. This is trueeven if the axis 140X is close to the axis 160. If the relation (1) didnot hold, and the axis 140X were close to plasma 120, some wafer points(for example, 134C) could pass through the plasma twice in a singlerevolution of drive 140. Other wafer points (such as 134F) would passthrough plasma 120 at most once in any revolution of drive 140.Therefore, the plasma processing would be less uniform unless the wafervelocity were doubled for points that pass through the plasma twiceduring a single revolution of drive 140.

R 2≧R 1  (2)

that is, the distance between axes 140X and 15OX (essentially the lengthof arm 150A) is greater than the distance between the axis 140X andwafer point 134C.

R 2−R 1<LP<R 2+R 1  (3)

LP+R 2≦LS  (4)

The relations (2), (3), and (4) allow every point of wafer 134 to beprocessed during a single revolution of drive 150 provided the velocityW2 is sufficiently low relative to W1.

The appendix at the end of this description gives equations that can beused to determine the angular velocity W2. The equations assume that W1is constant. The equations can be solved using known numerical methods.

Alternatively, W2 can be determined experimentally, for both constantand variable velocities W1, using known iterative techniques. Moreparticularly, when system 110 is being set up, test wafers are processedat some angular velocity W2 ₁ which is the first iterative approximationof the final velocity W2. In some embodiments, velocity W2 ₁ isconstant. Then the wafers are examined to determine which points wereprocessed too little relative to other points. For example, if theplasma processing is an etch process, the amount h₁(r) of the materialetched at different wafer points is examined, where r is the distancebetween the wafer point and axis 140X. Suppose that it is desired toetch away the amount h₀(r). (In many processes h₀(r) is independent ofr, that is, the same amount of material is to be etched away at everywafer point.) Then the velocity W2 ₂(r) at the second iterative pass isgiven by the formula:

W 2 ₂(r)=W 2 ₁(r)*h 1(r)/[h 0(r) 31 h 1(r)].

If additional iterations are desired, the velocity W2 at each subsequentiteration can be determined similarly (the more material is removed at agiven coordinate r during the previous iteration, the greater is thevelocity W2(r) during the next iteration).

These iterations are programmed into the control system 154. Inproduction, the control system 154 causes the system 110 to perform allthe iterations.

Because wafer points traveling at a greater linear velocity tend to havelonger trajectories through the plasma footprint 120F (see FIG. 5),uniform plasma processing can be achieved with less variation of theangular velocity W2 of drive 150. FIG. 6 illustrates this in oneembodiment for a 200 mm wafer. The horizontal axis D_(i) is the distancebetween a wafer point P on diameter 134D and the point 134C. The topcurve 610 shows the linear velocity V of the center of plasma footprint120F (axis 160) relative to axis 140X. The velocity units are chosen sothat V=1 at D_(i)=0. The velocity V was determined from velocity W2which in turn was determined from the equations in the appendix.

The bottom curve 620 in FIG. 6 shows the linear velocity V for a priorart apparatus having only the angle drive 150. The first drive 140 isomitted. In that prior art apparatus, the distance between the axis 15OXand the nearest wafer point 134C is R2+R1 where R2 and R1 are thedimensions for which the curve 610 was obtained. The linear velocity Vwas determined from angular velocity W2 which in turn was computed fromappropriate equations similar to those given in the appendix. Curves 610and 620 intersect at D_(i)=0, but the slope magnitude (and hence theacceleration) for curve 610 is smaller than for curve 620. The maximumacceleration (at D_(i)=0) for curve 610 is about two times smaller thanfor curve 620. Since the angular velocity W2 is proportional to V, theacceleration associated with W2 is also about two times smaller forcurve 610.

In some embodiments, the plasma system 110 corresponding to curve 610performs a back-side etch of wafers or individual dies to reduce thewafer or die thickness to 15-350 μm after the circuits on the wafers orthe dies have been fabricated. Such thin dies and wafers are suitablefor vertical integration modules in which multiple dies are stacked ontop of each other and then the whole stack is packaged. See PCT patentapplication WO 98/19337 “Integrated Circuits and Methods for TheirFabrication” published May 7, 1998 and incorporated herein by reference.The thin wafers and dies are fragile, and reducing the acceleration inthe plasma processing system reduces the possibility of damaging thewafers and dies and thus increases the yield.

In some embodiments, each holder 130 holds an individual die or someother article rather than a wafer.

In some embodiments, the direction of the W1 rotation (the rotation ofdrive 140) is changed during processing to achieve greater uniformity.The reason is, at high pressures (for example, atmospheric pressure) theplasma can be unstable when the wafer enters the plasma. Therefore, thewafer points at which the wafer enters the plasma are processed at alower rate than the rest of the wafer. In FIG. 7, for example, the W1rotation is clockwise. The path of plasma footprint 120F on wafer 134 isshown by hatching at 710. The wafer enters the plasma at the wafer edgeportion 134E-1, and exits the plasma at edge portion 134E-2. The areaadjacent to edge portion 134E-1 is processed at a lower rate. As thewafer rotates around axis 15OX, the wafer portion near the edge belowthe diameter 134D is processed at a lower rate.

Therefore, in some embodiments, the direction of the W1 rotation ischanged during processing. For example, in half of the W2 revolutions(around axis 150X) the W1 revolutions are in one direction (“positive”direction), and in the other half of the W2 revolutions the W1revolutions are in the opposite (“negative”) direction. This can be donein etches, depositions, and any other semiconductor andnon-semiconductor processes.

In some embodiments, the processing is divided into several stages. Forexample, in etches of semiconductor wafers, as the wafer becomesthinner, it heats up faster. Therefore, several stages are used.Different stages may have different processing parameters such asvelocities and cooling times (the times the wafer spends at Θ=0 forexample). In each processing stage, the processing parameters(velocities, etc.) are the same. Each processing stage is sub-dividedinto two sub-stages. In the first sub-stage, the W1 revolutions are inone direction, and in the second sub-stage, the W1 revolutions are inthe other direction. For example, the first stage may include fourhundred W2 revolutions. In the first two hundred W2 revolutions, the W1rotation is in the positive direction; in the next two hundred W2revolutions, the W1 rotation is in the negative direction. This isrepeated in the subsequent stages. The whole process may includethousands of the W2 revolutions, depending on the etching speed and theamount of the material to be removed.

In some embodiments, the direction of the W2 rotation also changesduring processing. Thus, some embodiments use any two or more of thefour combinations of the W1 and W2 rotations shown in the followingtable 1:

TABLE 1 Direction of W1 Direction of W2 Rotation Rotation positivepositive positive negative negative positive negative negative

In some embodiments, to improve uniformity, the articles are processedonly during one half of each W2 revolution. The reasons for this willnow be explained with reference to FIGS. 8-15. As shown in FIG. 8,wafers 134 sweep a ring-shaped (donut-shaped) area 810 20 bounded bycircles 234C and 234F. In FIG. 8, the outer edge 234F of ring 810 entersthe plasma 120, and plasma processing begins. The corresponding angle Θis shown as Θ1. No plasma processing takes place when Θ is between 0°and the value Θ1.

In FIG. 9, the inner edge 234C of ring 810 is leaving plasma 120, andthe plasma processing stops since the wafers do not intersect theplasma. The corresponding value of Θ is denoted Θ2.

Then Θ reaches the value of 360°−Θ2 (FIG. 10) as the ring 810 reaches aposition symmetric to that of FIG. 9 relative to vertical axis 220. Thering inner edge 234C re-enters the plasma 120, and the plasma processingresumes.

In FIG. 11, Θ=360°−Θ1. Outer edge 234F is leaving plasma 120, and theplasma processing stops again until the ring 810 re-enters the plasma asin FIG. 8.

Thus, each revolution of drive 150 includes two idle periods 1210, 1220(FIG. 12) in which no plasma processing takes place. The idle period1210 is between Θ=Θ2 and Θ=360°−Θ2. The idle period 1220 is betweenΘ=360°−Θ1 and Θ=Θ1 of the next revolution. The values Θ1 and Θ2, andhence the lengths of idle periods 1210, 1220, depend on the length R2 ofarm 150A (FIG. 13), the length D2 of arm 140A, and the diameter D3 ofwafer 134. In some embodiments, the lengths R2 and D2 are chosen toaccommodate a variety of wafer sizes in the same carousel. Thus, in someembodiments, R2=175 mm, and D2=200 mm. D3 ranges from 4 inches to 8inches, that is, from about 100 mm to about 200 mm. With suchdimensions, the angular length of idle path 1220 is significantly largerthan the length of path 1210.

In other embodiments, the dimension D2 is increased to accommodatelarger wafers (for example, over 300 mm diameter). Increasing the lengthD2 also allows more wafers to be placed on the carousel, increasing thethroughput. In many cases, path 1220 is longer than path 1210.

This leads to processing non-uniformity. At the beginning of path 1210(Θ=Θ2) the plasma has just processed the inner edge 234C (i.e. 134C) ofthe wafers. At the end of path 1210 (Θ=360°−Θ2), the inner edge 234Cre-enters the plasma. Thus, the path 1210 represents the time that theinner edge 234C is allowed to cool before being re-processed. Similarly,path 1220 represents the time that the outer edge 234F is allowed tocool before being re-processed. If the path 1220 is longer, the outeredge cools more, and hence is processed at a slower rate.

To even out the processing rates, in some embodiments the W2 rotation(the rotation of drive 150) is slowed down in path 1210 to allow theinner edge 234C to cool more. However, in path 1210, some portions ofthe carousel (including the arms 140A and, perhaps, the drive 140) areexposed to the plasma. Therefore, the carousel lifetime is reduced bythe heat and active chemicals carried by the plasma.

In other embodiments, the W2 revolutions are accelerated sharply at Θ=Θ2(FIG. 14). The linear velocity of the wafers increases, for example, by100-150%, and then sharply returns to 0 or some other suitable value (asneeded for cooling) when Θ is near 360°. Thus, the wafer processing issubstantially prevented when Θ is between 180° and 360°. Consequently,the cooling is more uniform, and the processing uniformity is increased.

In FIG. 15, the angular velocity W2 is increased for Θ between 0 and180° to prevent wafer processing. The wafer is processed between 180°and 360°.

The above embodiments illustrate but do not limit the invention. Theinvention is not limited to any particular shape or size of opening114-O or plasma footprint 120F. In some embodiments, the shape anddimensions of opening 114-O vary during processing. In some embodiments,the velocity W1 varies during processing. In some embodiments, drive 140is omitted. Plasma source 114 moves radially along a rotation radius ofdrive 150, and the opening 114-O rotates at the same time so that theplasma footprint 120F moves relative to the wafer as shown in FIG. 5.Some embodiments process articles other than semiconductor dies orwafers, for example, surgical instruments or machine tools. Otherembodiments and variations are within the scope of the invention asdefined by the appended claims.

Appendix

W 2(t)=∂Θ/∂t

∫₀^(T)W2(t)  t = 2πP(r1) = (1/2) * π * r1 * ∫₀^(T)∫₀^(2π)p(ρ(β), ϕ(t, β))  t  β, R1 ≤ r1 ≤ LS

 ρ(β)=(R 2 ² +r1²−2*R 2*r1*cos(β))^(½)

φ(t,β)=Θ(t)+cos⁻¹((R 2 ²+ρ²(β)−r1²)/(2*R 2*ρ(β)))

where:

t is time; T is the duration of one revolution of second drive 150;

P(r1) is the desired process result on the surface of the wafer 134along the radius r1 of the first drive 140;

p(ρ(β),φ(t,β)) is the distribution of the plasma treatment intensitywithin the plasma footprint 120F at the surface of wafer 134 at a pointhaving polar coordinates (ρ,φ) in the polar coordinate system having anorigin on the axis 150X;

β is the angular position of the arm 140A relative to any predetermineddirection in the plane of wafer 130 (i.e. perpendicular to rotation axes140X, 15OX).

What is claimed is:
 1. A method for processing an article with plasma,the method comprising: generating plasma; and moving the article throughthe plasma; wherein in a first period of time, the article motionthrough the plasma includes a rotational motion around a first axis in afirst direction; and in a second period of time, the article motionthrough the plasma includes a rotational motion around the first axis ina second direction opposite from the first direction; wherein therotational motion around the first axis in the first and second periodsof time includes rotating the article around the first axis as the firstaxis rotates around a second axis; wherein in the first period of time,as the article rotates around the first axis in the first direction, thefirst axis rotates around the second axis in a third direction, and inthe second period of time, as the article rotates around the first axisin the second direction, the first axis rotates around the second axisin a fourth direction opposite from the third direction.
 2. The methodof claim 1 wherein the first and second axes do not intersect thearticle.
 3. A method for processing an article with plasma, the methodcomprising: generating plasma; and moving the article through theplasma; wherein in a first period of time, the article motion throughthe plasma includes a rotational motion around a first axis in a firstdirection; and in a second period of time, the article motion throughthe plasma includes a rotational motion around the first axis in asecond direction opposite from the first direction; wherein therotational motion around the first axis in the first and second periodsof time includes rotating the article around the first axis as the firstaxis rotates around a second axis; wherein in the first period of time,as the article rotates around the first axis in the first direction, thefirst axis rotates around the second axis in a third direction, and inthe second period of time, as the article rotates around the first axisin the second direction, the first axis rotates around the second axisin the third direction; wherein during the rotational motion in thefirst direction, the article enters the plasma at a first edge portionof the article and exits the plasma at a second edge portion of thearticle; and during the rotational motion in the second direction, thearticle enters the plasma at the second edge portion and exits theplasma at the first edge portion.
 4. A method for processing an articlewith plasma, the method comprising: generating plasma; rotating anarticle around a first axis, and rotating the first axis around a secondaxis, so that the article gets processed with the plasma; wherein duringone half of each revolution of the article around the second axis, thearticle is processed with the plasma, and during the other half of eachrevolution around the second axis, processing of the article with theplasma is substantially prevented.
 5. The method of claim 4 wherein theprocessing of the article with the plasma is substantially prevented byincreasing the velocity of the article while moving the article throughthe plasma.
 6. The method of claim 4 wherein the article is asemiconductor wafer.
 7. A method for processing an article with plasma,the method comprising: generating plasma; and moving the article throughthe plasma; wherein the article motion through the plasma includes afirst rotational motion of the article around a first axis as the firstaxis performs a second rotational motion around a second axis; whereinat least one of the first and second rotational motions changesdirection during the processing; and wherein the first axis does notintersect the article.
 8. The method of claim 7 wherein the plasmaprocessing takes place at atmospheric pressure.
 9. The method of claim 7wherein the article is a semiconductor wafer.
 10. A method forprocessing an article with plasma, the method comprising: generatingplasma; and moving the article through the plasma; wherein the articlemotion through the plasma includes a first rotational motion of thearticle around a first axis as the first axis performs a secondrotational motion around a second axis; and wherein at least one of thefirst and second rotational motions changes direction during theprocessing; wherein a plasma footprint on the article is smaller than anarticle surface processed with the plasma.
 11. A method for processingan article with plasma, the method comprising: generating plasma; andmoving the article through the plasma; wherein the article motionthrough the plasma includes a first rotational motion of the articlearound a first axis as the first axis performs a second rotationalmotion around a second axis; and wherein at least one of the first andsecond rotational motions changes direction during the processing;wherein the rotation of the first axis around the second axis alternatesbetween a first direction and a second direction opposite from the firstdirection, such that in a first period of time, the first axis rotatesaround the second axis in the first direction as the article rotatesaround the first axis in a third direction, and in a second period oftime, the first axis rotates around the second axis in the seconddirection as the article rotates around the first axis in the thirddirection.